In the art of papermaking, chemical materials exist for improving the strength of paper when wetted with water or aqueous solutions, including body fluids such as urine, blood, mucus, menses, lymph and other body exudates. These materials are known in the art as "wet strength agents" and are commercially available from a wide variety of sources.
The substantivity or effectiveness of many cationic wet strength agents is limited by low retention of the wet strength agent on the cellulose fiber. Much of the applied chemical may not be retained on the fiber, but remains in solution or is washed off after application, for there are relatively few anionic sites on the cellulose surface to attract the charged wet strength agent, and in some cases there may be a large number of anionic sites on colloidal particles or other particles in the fiber suspension which may adsorb a large portion of the wet strength agent, limiting its effectiveness in increasing wet strength. Likewise, the presence of anionic additives or agents in the pulp has a deleterious effect on the efficiency of cationic wet strength agents. This adverse effect can be reduced by adding "cationic promoters" or other cationic additives to the stock, as is known in the art of papermaking, to help neutralize excess anionic sites on colloidal particles or "anionic trash" in the suspension, to allow more of a subsequently added cationic wet strength resin to attach to the fiber surface and not to be preferentially absorbed onto non-fiber components. Such additives can, for example, be cationic promoters such as polyethyleneimine with a cationic charge of about 0.75 to 3.5 milliequivalents/gram, quaternized polyamines, such as polydiallyidimethylammonium chloride, or cationic starch. Commonly used cationic resins include polyquaternary amines and are available from Cytec Industries under the trade names CYPRO 514, 515, 516. Cationic promoters are added to the stock in advance of the wet strength resins to ensure adequate mixing and adequate contact with the fibers. When used, the cationic resins are generally used in an amount of about 1 to 10 pounds per ton or 0.05 to 0.5%. The cationic promoter can be used at 0 to 0.5 wt %; typically the resins are used in an amount of about 0.02 to 0.3 wt % and preferably 0.1 to 0.2 wt %. The manufacturer of the promoter will typically recommend a pH for its use. The Cypro resins, for example, are effective over a pH of about 4 to 9.
However, the use of cationic promoters does not increase the number of anionic sites on the fiber surface itself, and may decrease the number of such sites, such that the intrinsic potential of the cationic wet strength agent to increase wet strength is still limited by inadequate attachment sites on the cellulose surface. What is needed, therefore, is an improved means of increasing the wet strength performance of paper prepared with cationic wet strength agents through the addition of anionic sites on the cellulose fiber. (The extent of anionic sites on the cellulose can be measured in terms of the carboxyl group content of cellulose, which is typically measured to be about 2 to 5 milliequivalent per 100 grams of cellulose, or higher.)
While the use of fiber reactive agents to enhance the efficiency of wet strength agents is not known, fiber reactive agents are known in the art, particularly for treatment of textiles. In particular, anionic fiber reactive dyes are well known in the art. By reactive dyes are meant the customary dyes that form a covalent bond with cellulose, e.g. those listed under the heading "Reactive dyes" in the Colour Index, Vol. 3, 3rd Edition (1971), on pages 3391-3560, and in Vol. 6, revised 3rd Edition (1975), on pages 6268-6345. Fiber reactive dyes contain functional groups with react with the hydroxyl groups of cellulose to form covalent bonds, and further contain anionic groups such as sulfonic groups. Monochlorotriazinyl reactive dyes are one exemplary class. Other fiber-reactive groups may be, for example monofluorotriazinyl, dichlorotriazinyl, dichloroquinoxalinyl, trichloropyrimidyl, difluorochloropyrimidyl, the .alpha.-bromoacrylamide group or the .beta.-oxyethylsulphuric acid ester group, as disclosed in U.S. Pat. No. 4,155,707 issued to Franceschini et al., May 22, 1979, herein incorporated by reference. Many commercial dyes are stilbene derivatives and particularly are derivatives of 4,4'-diaminostilbene-2,2'-disulfonic acid, sometimes known as flavonic acid. Other fiber reactive dyes of importance are disclosed in U.S. Pat. No. 5,432,266 issued Jul. 11, 1995 to Herd and Roschger; U.S. Pat. No. 4,402,703 issued Sep. 6, 1983 to Panto and Kaswell; all of which are herein incorporated by reference.
In addition to fiber reactive dyes, fiber reactive fluorescent whitening agents and optical brighteners are known which employ reactive groups such as the chloro- or fluoro-s-triazinyl radical or a 5-chloro-2,6-difluoro-4-pyrimidinyl or 5-chloro-6-fluoro-4-pyrimidinyl radical; and other moieties known in the art of fiber reactive dyes, coupled with UV absorbing structures such as stilbene derivatives. Fluorescent whitening agents do not absorb light strongly in the visible spectrum, being substantially colorless in visible light, but do absorb ultraviolet light (e.g., in the wavelength range of about 300 to about 400 nm) and re-emit the energy absorbed as visible light, typically blue, thus increasing the apparent brightness of the material and helping to overcome a possibly yellow appearance. If excessive doses of fluorescent whitening agents are used, the material may no longer appear white but may have a blue, purple, or green tinge. Typical fluorescent whitening agents are derived from stilbene compounds, cumarins, benzocumarins, pyrazines, pyrazolines, oxazines, dibenzoxazolyl or dibenzimidazolyl compounds and naphthalimides, with stilbene among the most common. Exemplary fluorescent whitening agents are disclosed in U.S. Pat. No. 3,951,588, issued Apr. 20, 1976 to Perrin et al.; U.S. Pat. No. 4,140,852, "Triazinyl Styryl-Benzoxazole Fluorescent Dyestuffs," issued Feb. 20, 1979 to Eckstein and Harnisch; U.S. Pat. No. 3,951,588, "Process for Dyeing and Printing or Optical Brightening of Cellulose," issued Apr. 20, 1976; U.S. Pat. No. 4,228,071; "Triazine Containing Fiber-Reactive Disazo Dyestuffs," issued Oct. 14, 1980 to Riat and Seltz; U.S. Pat. No. 4,134,724, issued Jan. 16, 1979 to Thompson et al.; and U.S. Pat. No. 4,141,890 issued Feb. 27, 1979 to Hegar and Back, all of which are herein incorporated by reference.
While many optical brighteners or whitening compounds used in the art of papermaking have anionic groups that might be able to form bonds with cationic wet strength additives, fiber-reactive whitening compounds have not been used in a manner that can provide improved wet strength in paper or improved retention of wet strength compounds. Indeed, when possible interactions between whitening compounds and wet strength agents have been considered, it has been taught that the whitener should be added to the pulp after the wet strength agent has been added, as in German Patent No. DE 1,283,083, published Nov. 14, 1968 by H. E. Gottgens and H. Tretter of Bayer AG, in which case no improved retention of the wet strength agent by means of increased anionic sites on the fiber can be expected. Further, it has been taught that cationic polymer additives hinder the brightening effect of fluorescent whitening additives and can increase the apparent yellowness of a sheet by quenching fluorescence (B. W. Crouse and G. H. Snow, "Fluorescent Whitening Agents in the Paper Industry," Tappi J., Vol. 64, No. 7, July 1981, pp. 87-89). The possibility of negative interactions between cationic agents and fluorescent whitening agents was also recognized by H. Geenen in "Possibilities for Improving Paper Brightness," Wochenblatt Papierfabr., vol. 114, no. 2, end January 1986, pp. 41-42.
Reactive optical brighteners and fluorescent whitening agents are now rarely if ever used in the paper industry because of the tendency to hydrolyze when added to aqueous suspensions and because of other problems associated with the reactivity of the compounds. Indeed, as of 1998, it appears that no supplier of dyes and dyestuffs produces commercially available fiber reactive optical brighteners for use in the paper industry. Thus, the potential benefits of fiber reactive optical brighteners for papermaking properties appear not to have been recognized.
While fiber reactive forms may not be in use in the paper industry, non-reactive fluorescent whitening agents and optical brighteners are widely used. While the major uses are probably for improving the brightness of coated and uncoated printing and writing papers, one possible use is in the prevention of photoyellowing of high-yield fibers, particularly TMP and BCTMP. The lignin compounds in high-yield pulps can rapidly degrade to produce a yellow color upon exposure to UV light. The yellowing of newspaper, which usually comprises TMP or groundwood, is well known, but there are many other products for which yellowing is problematic. Paper towels and bath tissue, for example, can become yellow due to the ultraviolet component of ordinary fluorescent lights while sitting on the shelf of a grocery store.
In theory, if a compound absorbs UV energy, it may prevent the UV energy from causing reactions in the lignin that lead to yellowing. Ideally, the UV absorbing agent should be able to continually absorb UV energy and re-emit a portion of it as fluorescence rather than decomposing rapidly due to the energy absorbed. For this reason, fluorescent whitening agents appear to have promise in shielding high-yield pulps from the yellowing caused by UV light, and in hiding the yellowish tinge of such pulps through the addition of blue light from the fluorescence. While stilbene structures in high-yield pulp contribute to yellowing, especially in pulps bleached with peroxides (see "Reactive Structures in Wood and High-Yield Pulps; Daylight-Induced Oxidation of Stilbene Structures in the Solid State," by L. M. Zhang and G. Gellerstedt, Acta Chem. Scand. 48, no. 6: 490-497, June 1994), stilbene derivatives that function as UV absorbers may be able to reduce yellowing of high-yield paper by shielding lignin from UV. However, for durable materials or products intended for long-term use or long shelf lives, there is the risk that stilbene additives themselves will lead to yellowing with time, typically due to oxidative reduction of the double bond in the stilbene group. The decomposition of the stilbene derivative can lead to production of yellow chromophores or other undesired products. (Thioglycolic acid is known to cause some degree of photostabilisation of natural stilbene compounds in high-yield pulps, but poses other difficulties associated with sulfur compounds and with cost.) For this reason, it may be desirable for some products to avoid the use of stilbene derivatives altogether. For example, products comprising high brightness fibers such as bleached kraft fibers may be unsuited for the use of fluorescent whitening agents or stilbene derivatives in particular, if such compounds may degrade to give yellow chromophores. Further, in some countries, optical brighteners or fluorescent whitening agents are not permitted in paper packaging which may contact food. Further still, for some products and materials it is desirable that the hue or shade or white not be affected by the presence of UV light (i.e., the degree of whiteness or brightness is similar for both incandescent and fluorescent lights or incandescent light and sunlight). In such cases, the paper web should be substantially free of fluorescent whitening agents such that the web does not fluoresce in UV light. Thus, fluorescent whitening agents may not be desirable for all grades, but may be suited for high-yield grades, especially for disposable products where short-term protection from photoyellowing is needed.
Thus, in terms of fluorescent whitening agents, some applications may benefit from a synergistic use of fluorescent whitening agents that also promote improvements in non-optical properties of the web such as wet strength, while other applications may not be benefited by use of fluorescent whitening agents.
Therefore, an object of the present invention is to increase the number of anionic sites on the surface of papermaking fibers by pretreating the fibers, thus increasing the substantivity of subsequently added cationic wet strength agents that form covalent bonds with the cellulose. A further object of the present invention is to provide a means of increasing both the wet strength and the brightness of a web, particularly a web comprising high-yield papermaking fibers. A further object of the present invention is substantially increasing the wet strength of paper that can be achieved with a given dose of wet strength agent.